This F31 fellowship application outlines the training and research plan that will prepare Katherine Best for a future career as an independent investigator with expertise in sophisticated imaging of inflammatory pathologies. This project studies scar-mediated healing following acute tendon injury and repair, which is characterized by the deposition of abundant scar tissue. As the remodeling stage of healing predominates, the injured tendon slowly increases in strength over several months while never recovering the tissue?s original mechanical properties. Additionally, the exuberant scar tissue deposition can hinder full range of motion (ROM) in specific tendons such as the rotator cuff and flexor tendons. Tendon injuries are characterized by an inflammatory response, which recruits macrophages (M?) that express pro-inflammatory cytokines such as IL-1? and TNF-?. Inflammation is a necessary part of healing as M? can stimulate collagen deposition by myofibroblast cell populations. However, excessive inflammation can result in overabundant deposition of scar tissue, leading to the reduced strength and inhibited ROM seen in tendon. Little is understood about the inflammatory mechanisms that promote scar formation during tendon healing. Preliminary studies suggest that the pro-inflammatory canonical NF-?B signaling cascade, which is activated by IL-1? and TNF-?, is up-regulated following tendon injury and that global knockdown of this pathway decreases scar deposition while maintaining tendon biomechanics. These studies lead us to our central hypothesis that canonical NF-?B signaling is a pro- inflammatory mediator of scar tissue deposition in tendon healing and that tendon-specific knockdown of this pathway will improve tendon outcomes in disease. To assess the contribution of canonical NF-?B signaling in tendon healing, our lab?s well characterized tendon injury model, which consists of an acute flexor tendon injury and repair, will be used. We will define the temporal and spatial activation of canonical NF-?B to better understand the stages of healing affected by this pathway and to identify cell populations activating canonical NF-?B, specifically tenocytes (Aim 1). Data suggests that tenocytes can activate canonical NF-?B signaling in the presence of inflammatory cytokines in vitro. Therefore, we will induce knockdown of NF-?B signaling by inducing IKK? knockout in tenocytes using genetic mouse models (Aim 2A). We anticipate that tenocyte-specific knockdown of canonical NF-?B will be beneficial due to a decreased recruitment of M?s. We will assess tenocyte-M? communication to determine if interactions between these cell types in an inflammatory environment directly contribute to scar tissue formation (Aim 2). The ultimate goal of this proposal is to examine the negative effects canonical NF-?B enacts on acute tendon injuries and to examine altered phenotypes following tenocyte-specific knockdown of this signaling pathway. The completion of this training plan will provide Katherine Best with opportunities to develop her technical, communication, leadership, and mentorship skills and will prepare her for a successful career as an independent investigator of inflammatory pathologies.